Grid-connected renewable energy systems have experienced a rapid growth in the U.S. recently. Both wind and photovoltaic energy production have almost doubled in the last several years requiring new energy storage systems. As an example, the annual growth rates in the U.S. were 25% and 74% for wind and photovoltaic energy, respectively, in 2011 over 2010. Due to the variable and stochastic nature of renewable sources this energy is difficult to manage, especially at high levels of penetration. The current lead-acid and flow batteries that are being used in grid-connected renewable systems are not cost effective and reliable enough to be integrated in large grids. New storage solutions would ultimately need to be scaled to tens of gigawatts of power with tens of gigawatt-hours of energy distributed across the grid, to address the minutes-to-hours power firming and smoothing needed for renewable energy generation nationwide.
Recently, Li-air batteries have been attracted much attention because of the possibility of extremely high energy density. The theoretical energy density of the batteries can be over 3,000 Wh/kg which is more than 10 times greater than that of Li-ion batteries. Although traditional Li-air batteries have an extremely large theoretical energy density, they suffer from several drawbacks: (1) the Li2O2/Li2O discharge product deposits on the air side of the electrode reducing the pore size and limiting the access of O2 into the cathode. The discharge products deposit mostly near the air side of the electrode because the O2 concentration is higher on this side. This inhomogenous deposition of reaction products severely limits the usage of cathode volume, which limits the maximum capacity and energy density of the battery; (2) the cyclability and energy efficiency of Li-air batteries are poor due to the lack of effective catalysts to convert solid Li2O2/Li2O discharge products into Li ions; and (3) the current and power densities of Li-air batteries are much lower compared to conventional batteries due to the extremely low oxygen diffusion coefficient in liquid solution.
There are some efforts to improve the cyclability of Li-air batteries with most research focusing on the development of catalysts which can effectively accelerate the oxygen reduction process and reduce recharge overvoltage. The poor reversibility of Li-air batteries is due to the formation of solid oxide discharge products which are difficult to reduce and decompose into Li-ions and oxygen within the electrolyte's stable potential. Improved catalysts could reduce the reduction potential but could not effectively reduce all solid oxide products deposited in a highly porous electrode. The most significant challenge to rechargeability of Li-air batteries is the formation of solid discharge products.